I used epoxy to glue the back assembly. Because the spindle orientation is intentional, I wanted to prevent them from spinning. I am glad I used epoxy because it took a while to get everything oriented and clamped. It was very awkward to orient pipe clamps to hold the crest and the seat simultaneously. If I do this again, I will consider gluing blocks onto the bottom just to be able to pull it tight.
The only thing left is to shape the 'horns'. I originally intended to try my hand at a false miter, but I think I may just opt for a simpler, blended transition...